[MUSIC] Welcome back to my Coursera class. I'm about to finish second week by showing the last session. It's about carbohydrate absorption. Okay, how The digestive glucose and other monosaccharides can be absorbed from the lumenal side of intestine into our blood system. That's the main topic for today's session. We are looking at the small intestine's structures. As I said, intestinal epithelial cells, these brush border cell is lined up and making a long and highly invaginated sheet In this kind of primary barrier of your intestine. So, and this is brush border cells. Let's say that here is lumenal side, and this is brush border cells, and making a epithelial layer. And each cells, they are very, very tightly linked to protect the import of unnecessary molecules, and they highly control the absorption of nutrient. In particular, in this case, let's say glucose. Glucose is highly digested, and they have to go through this brush border of membrane system, but how? So let me give you a introduction of cellular membrane. What is cellular membrane? Cellular membrane is a sheet-like structure forming a continuous physical barrier around the cell. And obviously, the main function for cellular membrane is keeping our cells safe and allowing nutrients and very important chemicals to get into our body, right? Just a point. So what's the structure of cellular membrane? Basically, phospholipid, phospholipid bilayer forming a sheet-like structure. And on top of this lipid-based membrane or structure, there are many, many proteins embedded. Of course, the plasma membrane. So, some molecular transport through a cellular membrane. As I said, phospholipid, very, very hydrophobic carbon tails, hydrocarbon tails, and some phosphate-containing carbon backbones, this bone structure. And this tail structure is hydrophobic carbon tails. The thing is let's say here is extracellular on top of this membrane and intracellular of cells. So target molecules just passively can come in and out throughout the cellular membrane structure, throughout this phospholipid bilayer. But the thing is only small and hydrophobic molecules can passively and nicely diffuse across a phospholipid bilayer. And this is very, very important biochemical feature of cell membrane. Only small-sized hydrophobic molecules can, so simply can be diffused throughout cellular membrane system. So this is like some examples for molecular transport across the phospholipid bilayer. Let's say that here is the exterior, some classes of molecules like gas, CO2, nitrogen gas, or oxygen. The size is very, very small and their diffusion rate is very high. They can very, very easily goes through the cellular membrane. There is no big problem in terms of molecular transport, in particular, these gases. What about small-sized and uncharged even polar but no charged and very small molecules like ethanol or water? Let's say that they're a couple of highly efficient diffusion. Compared to gases, ethanol and water, relatively slow in terms of diffusion rate. But still, these small-sized uncharged polar molecules can go into the cytoplasm of intestinal epithelial cells. What about glucose? This is the problem, what about glucose? Glucose, the size is big, and glucose is hydrated. That means quite polar. Do you think these glucose molecules can just be diffused into the epithelial cells? That's not true. This is impossible. There is no diffusion. What about ions? In terms of size, this is very, very small. But because of those charges on top of those ions, it is impossible to penetrate across this hydrophobic phospholipid bilayer. And even amino acid ATP, those bulky and charged and polar molecules, they cannot get into the cells. Okay, we are in the middle of glucose absorption. How this glucose, during the last four sessions of second week, we studied about how we feel about hunger, how we start consuming food, and those carbohydrate polymers digests these salivary amylase and pancreatic amylase, and maltase, lactase, whatever. Now, we just digested those carbohydrate polymers in the form of glucose monomer, it's time to import this glucose inside our body. The first barrier is intestinal epithelial cell. But glucose cannot just simply diffuse into the epithelial cells. Then what types of molecular machinery required? That is facilitated diffusion. This process requires specialized protein called transporter. Transporter, you are looking at this diagram. Okay, transporter, by definition, transporter mediate the movement of target molecules by physically binding to and moving the substance across the membrane. This is one of good example. This transporter can recognize and bind to these green target molecules, and there is a huge conformation or changes from one environment. Let's say that this is high concentration, high concentration to low concentration. These green molecules can be transported. This is mediated by transporter, and there is a specificity. Specific target molecules that transport is mediated by a specific transporter, which is membrane-bound enzyme. And these transporters are also recognized as carriers, or permeases, or translocators. The thing is depending on the number of molecules and directional molecular transport, we can subdivide this class, sometimes uniporter. That means only one type of molecules can be transported. Or symporters, that means two different molecules can be transported at the same time to the same direction. Antiporters is opposite, two different molecules can be transported in opposite directions. All right, so as I said, glucose cannot be just simply diffused into the epithelial cells, intestine epithelial cells, and that transport, monosaccharide transport, is mediated by this process, facilitated diffusion. This is a summary slide and very important last slide. Absorption of glucose and other monosaccharide transport across the intestinal epithelium. Let's say that here is lumenal side. Outside, this is intestinal cells, all right? Let's say they're fructose. Out of this sucrose degradation, fructose is just release in the lumenal side of small intestine, there is fructose transporter. And then, get into the epithelial cells. And then, theories, fructose transporter. On the other side of intestinal epithelial cells, and then this fructose can get into the blood circulatory system. What about the glucose and galactose? When glucose and galactose, they are ready to undergo molecular transport in the epithelial brush border membrane, there is specialized transporters. And those transporters, glucose and galactose, can be co-transported with sodium. And sodium and glucose, or sodium and galactose, can be transported inside this intestinal epithelial cells. And then on the other side, this is like a Base membrane structure. Another transporter molecule can selectively Transport glucose or galactose into the circulatory system. And this locally increased sodium ions should be pumped out, and then at the same time, potassium come in, and this is kind of review, sodium potassium ATPase, the previous session [INAUDIBLE]. All right, so this is how the fully digested monosaccharide and glucose molecules can be transferred from the outside to the inside, and finally, throughout the capillary of the circulatory system, and finally arrive at the surface of target cells. And basically, let's say the glucose molecules over here and the glucose molecules from bloodstream into the inside the cells, again, the same logic and same biochemical principle applied, transporters. Mediate glucose uptake inside the cytoplasm of a target cell.
